This invention relates to circuits for driving gas discharge lamps, and particularly, though not exclusively, to circuits for driving fluorescent lamps.
In a typical prior art circuit for driving a plurality of fluorescent lamps, the lamps are driven from a high-frequency oscillating circuit powered, via a rectifier and an inverter, from an AC voltage supply, e.g. an electric utility mains.
It is well recognized that although the voltage level of an electric utility mains is nominally fixed at known level, e.g. 120 V, the actual voltage level of the mains typically varies in dependence on conditions such as variations in the power drawn by other users from the mains and variations in the available generating capacity. In order to provide uniform lamp operation in the face of variations in the actual mains supply voltage it is usual to control the power drawn from the mains by drawing less or more current so as to produce a constant light output over a wide range of operating conditions. Indeed with the recent move throughout the lighting industry to change from magnetic ballast circuits to electronic ballast circuits for driving fluorescent lamps and with the ease of incorporation of electronic power control in electronic ballast circuits, it is widely considered a mark of good ballast design for a ballast circuit to provide steady light output over a wide range of supply voltage variation both above and below the nominal supply voltage by reducing or increasing the current drawn from the mains supply respectively.
However, while it is desirable that a ballast circuit should provide steady light output by reducing the current drawn if the supply voltage varies over a wide range above its nominal value, the inventors hereof have realized that it may not be generally advantageous for a ballast circuit to increase the current drawn if the supply voltage varies over a wide range below its nominal value. One circumstance in which a fall in mains supply voltage below nominal value may occur is when there is an unexpected drawing of power from other users of the mains. Another circumstance in which a fall in mains supply voltage below nominal value may occur is when the electric utility deliberately reduces the mains voltage; this circumstance may arise if the electric utility has reached a maximum in its generating capacity and needs to reduce the power drawn from the mains in order to maintain the safety of its generating equipment, e.g. to maintain the temperatures of its transformers at a safe level. If the electric utility deliberately reduces the mains voltage in order to reduce the amount of power drawn, such safety action will be rendered ineffective if circuits such as lamp ballasts which are drawing power from the mains respond by increasing their current drawn so as to maintain constant the power which they draw.